1. Field of the Invention
This invention relates to complex alternating current and direct current signal detectors and, more particularly, to the detection of superimposed party line telephone ringing signals.
2. Description of the Prior Art
In subscriber loop carrier systems which service multiparty telephone lines, it is necessary to transmit the selective ringing signals through the carrier system. As shown in the copending application of R. J. Canniff-M. T. Manfred, Ser. No. 659,369, filed Feb. 19, 1976, it is therefore necessary to detect the multiparty superimposed ringing signals at the central office terminal of the carrier system. The detected ringing signals can then be encoded for transmission over the carrier system and used at the remote terminal of a carrier system. One such encoding scheme is disclosed in the aforementioned application of Canniff and Manfred.
Superimposed ringing signals comprise a nominal 20 Hz alternating current signal having a nominal peak voltage of approximately 120 volts. This signal is superimposed on a direct current battery voltage of a selected polarity. In order to distinguish between four different telephone subscribers connected to the same party line, the 20 Hz signal is applied selectively to either the tip or ring conductor and, furthermore, is superimposed on either a positive or a negative battery voltage.
Due to the large variation in actual waveforms and voltages of ringing signals and, moreover, due to the large variation in central office battery voltages, it is difficult to detect, both accurately and unambiguously, the ringing signals supplied by the central office. Timing errors in the detected signal could result in short ringing bursts (or bell taps) being applied to the ringer of the wrong subscriber, thus presenting a substantial interference with proper telephone service. This problem is greatly aggravated for revertive ringing situations where one party on a party line calls another party on the same party line. The normal procedure for such a case is to have the calling party hang up the telephone immediately after dialing the called party. The central office then alternately applies two selective ringing signals to the party line, first for the called party, and then for the calling party. When the called party picks up the telephone, ringing is tripped and the calling party, noting the termination of ringing, also picks up the telephone and a conversation can take place.
It will be noted that revertive ringing requires rapid switching of ringing signals between the tip and ring conductors and/or simultaneous rapid reversals of the polarity of the central office battery connections. It is particularly important in this situation to detect properly both ringing signals so as to ring only the calling and the called party's telephones. The wide latitude in voltages and waveforms of central office ringing signals makes possible wide variations in the timing of the detector outputs during revertive ringing. Correct and unambiguous detection of these ringing signals without timing overlaps is important to the proper operation of the party line carrier system.